5110cn erratic envelope jam errors

I've got a 5110cn now out of warranty. It was replaced twice for toner problems and now I am having jamming errors on envelopes.  Regular paper jams are very rare.  Everything is set correctly on software - envelope, size, thickness, landscape, etc. We have replaced all our envelopes with one that was working perfectly, no wrinkles and no jams.  In the past few months it has become unbearable. We run a few hundred a week at the most and it's now taking us hours of frustration.

The envelope feeds exactly 2" and then we get a jam error. It always feeds exactly 2" when the error happens. This happens erratically and sometimes it's on the first, sometimes we'll get 20 before the error.  I have cleaned and checked the rollers as best possible, see no obstructions at all.  I've read that some people have corrected this on other Dells by bending forward the sensor. Exactly where is the sensor??  It feels exactly like a sensor problem, not an actual jam at all. The first roller does not seem to turn less than it should or spin more. I've had lots of printers and copiers and this type of problem ususally comes down to worn rollers or sensor.  This unit has under 35K copies (98% 8 1/2 x11 paper) so worn rollers are unlikely.

Any and all suggestions appreciated.

Doug